Environmental Variability and Climate Change

period, establishment of forest ecosystems lagged
behind temperature changes by up to two thousand
years, depending on altitude. Although past climate
changes are marked by disruptions of ecosystem
composition, the responses vary greatly from species
to species and between different types of ecosystem.
In more recent times, the impact of human activities
on many ecosystems has been both to fragment and
to impoverish. These changes are likely to exacerbate
the disruption of ecosystem function and complicate
understanding of ecosystem response to future
climate change.
Can biota adjust to ongoing and
projected rates of climate change?
The rate and magnitude of projected climate change will
cause major disruptions of present-day ecosystems and
species distribution. Individual species, for example, will
need to shift their ranges at rates that are 10-40 times
faster than has been observed since at least the last ice age
if they are to maintain equilibrium with projected climate
changes. It seems unlikely that such adjustments will be
possible given the presently fragmented nature of most
landscapes. In light of these potential lags in the biotic
response to climate change, ecological instability and
the occurrence of natural disturbances, like wildfi res, will
probably increase.
To what extent have past ecosystem changes
affected climate?
Most of the evidence for climate feedbacks from the
terrestrial biosphere comes from model simulations
and data-model comparisons. Model simulations
often best match paleodata when feedbacks from
vegetation and soils are included. The rapid pace of
warming at high latitudes in Eurasia at the end of
the last glaciation, for example, involved changes
in the amount of solar energy refl ected into space
associated with the transformation of land cover
from permanent snow to soils and forests. The
maintenance of vegetation in the Sahara several
thousand years ago also probably involved climatic
feedbacks. The vegetation seems to have been a
crucial element in driving the climate to a state
which then supported more vegetation. In more
recent times, some feedbacks may have resulted from
deforestation as demand for pasture and farmland
has increased.
Multiproxy climate reconstruction and ecosystem response
Figure 18
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Multi-proxy reconstruction of temperature using pollen,
chironomid and diatom transfer functions developed
in Northern Scandinavia. Both pollen and chironomid
surface temperature estimates are within 0.1ºC of
the measured temperature. The red circles indicate
communities without any modern analogue. Although
the temperature decrease was subtle through the
Holocene, diatom communities sharply changed after
6000 years ago. Chironomid assemblages also varied
through time with non-analogues periods after
deglaciation. The striking non-analogues for chironomids
in the two most recent levels might indicate changes of
communities following fi sh introduction by humans.
source: Bigler et al (submitted) The Holocene
24 IGBP SCIENCE No. 3 PAGES – Ecosystem Processes
July Temperature (°C)
pollen
chironomids
diatoms
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thousands of years ago
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